Certain clinical and valve morphologic findings are described in 11 patients with hypertrophic cardiomyopathy (HC) complicated by infective endocarditis (IE) which produced severe mitral and/or aortic valve regurgitation necessitating valve replacement. All 11 patients had changes in the operatively excised valve(s) characteristic of IE which had healed, and 10 of the 11 patients had had clinical episodes characteristic of active IE which healed. The infection involved the mitral valve only in 7 patients, the aortic valve only in 3 patients, and both .valves in 1 patient. Study of the operatively excised mitral valves indicated that the vegetations, which had healed, had been located most commonly on the ventricular aspects of the anterior leaflet indicating that vegetation had formed at contact points of the anterior leaflet with mural endocardium of the left ventricular outflow tract. In all 11 patients the IE either worsened pre-existing valve regurgitation or initiated valve regurgitation, and led to worsened signs and symptoms of cardiac dysfunction necessitating valve replacement. In the 9 patients who survived 7 to 101 months after cardiac valve replacement operations, functional class improved. In summary, HC appears to be a predisposing factor to IE and patients with HC should receive prophylactic antibiotics during procedures which also predispose to IE.